Merge branch 'sg/bash-prompt'

Merge branch 'sg/bash-prompt'

* sg/bash-prompt:
  bash prompt: mention that PROMPT_COMMAND mode is faster
  bash prompt: avoid command substitution when finalizing gitstring
  bash prompt: avoid command substitution when checking for untracked files
  bash prompt: use bash builtins to check stash state
  bash prompt: use bash builtins to check for unborn branch for dirty state
  bash prompt: combine 'git rev-parse' for detached head
  bash prompt: combine 'git rev-parse' executions in the main code path
  bash prompt: use bash builtins to find out current branch
  bash prompt: use bash builtins to find out rebase state
  bash prompt: run 'git rev-parse --git-dir' directly instead of __gitdir()
  bash prompt: return early from __git_ps1() when not in a git repository
  bash prompt: print unique detached HEAD abbreviated object name
  bash prompt: add a test for symbolic link symbolic refs
  completion, bash prompt: move __gitdir() tests to completion test suite
  bash prompt: use 'write_script' helper in interactive rebase test
  bash prompt: fix redirection coding style in tests